1. Field of the Invention
The present invention relates to a surface acoustic wave convolver and a communication system using it wherein an convolution output is obtained by the use of non-linear interaction of a plurality of elastic surface waves.
2. Related Background Art
An elastic surface wave convolva has been recently noted for its importance as a key device in making the spread spectrum communication. Also, many applications as a real-time signal processing device has been considered and studied actively.
FIG. 1 is a schematic plan view showing an example of such a conventional elastic surface wave convolva.
In the same figure, a piezoelectric substrate 1 is provided with a pair of input interdigital transducers 2 and a central electrode 3 therebetween. The transducers 2 are electrodes for exciting a surface acoustic wave signal, while the central electrode 3 is an electrode for propagating the surface acoustic wave signal in opposite directions to each other and for taking out an output signal.
If a signal F(t)exp(j.omega.t) is applied to one of the transducers 2, and a signal G(t)exp(j.omega.t) to the other, two surface acoustic waves in opposite directions to each other EQU F(t-x/v)exp[j.omega.(t-x/v)] (1a)
and EQU G(t-(L-x)/v)exp[j.omega.(t-L-x)/v)] (1b)
will propagate on a surface of the piezoelectric substrate 1. Where v is the velocity of surface acoustic wave and L is the length of central electrode 3.
On this propagation path, a product component of above surface acoustic waves is produced due to non-linear effect, and integrated over a range of the central electrode 3 so as to be taken out. This output signal H(t) is represented by the following expression. ##EQU1## Where .alpha. is a proportional constant.
Thus, a convolution signal of two signals F(t) and G(t) can be obtained from the central electrode 3.
However, with such a constitution, as the efficiency is generally insufficient, as shown in FIG. 2 has been proposed by Nakagawa et al, in "Electronic Communications society journal" 1986/2, Vol. j69-C, No. 2, pp190-198. Note that the axis of coordinate y as shown in FIG. 2 was appended for convenience, not meaning the crystal axis of substrate.
In FIG. 2, 11 is a piezoelectric substrate, and 12, 13 are two input interdigital transducers for excitation of surface acoustic wave formed on the substrate 1, opposed to each other and spaced by an appropriate distance in the x direction. 14-1, 14-2, . . . , 14-n are waveguides formed on the substrate 11 extending in parallel in the x direction between the transducers 12, 13. And 15 is an output interdigital transducer formed on a surface of the substrate 11, spaced by an appropriate distance in the y direction from the above-mentioned waveguide.
In this elastic surface was convolva, if an electrical signal with an angular frequency .omega. is input to the transducers 12, 13 for excitation of surface acoustic wave, the surface acoustic wave of that frequency is excited, and propagates on the waveguides 14-1, 14-2, . . . , 14-n in the x direction but in opposite directions to each other, in which the elastic surface wave with an angular frequency 2.omega. propagating in the y direction may occur on the waveguides due to parametric mixing phenomenon. This elastic surface wave arrives at the output transducer 15 in which a convolution electrical signal for two input signals as above indicated can be obtained.
However, in the elastic surface wave convolva as shown in FIG. 2, if the interaction length (integral time) of signals is desired to be longer, the length of the waveguides 14-1-14-n must be increased. As the length of the output transducer is equal to that of the waveguides, the output transducer must be also lengthened naturally when the interaction length is increased.
Since the width of electrode digit for the output transducer can be determined by the frequency of convolution signal and the propagation velocity of elastic surface wave on the substrate, the line width becomes thinner if the input center frequency becomes higher.
For example, in a split waveguide convolva using a 128.degree. Y.multidot.X LiNbO.sub.3 monocrystal as the substrate, with an input center frequency of 200 MHz and an interaction length of 6 .mu.s, an electrode digit for output transducer has a line width of 2 .mu.m and a length of 20 mm.
There was a problem that the resistance of electrode digit for this transducer is about 2 k.OMEGA. per line, whereby the convolution efficiency is reduced due to the resistance of this electrode digit.
Also, since the conventional output interdigital transducer as above described has a thin width of electrode digit of several .mu.m, while the length is as long as several mm to several tens mm, there was a problem that the fabrication was difficult and the yield was bad.